Coal is a low cost fuel that is abundant in the United States, as well as other countries around the world. Both the price and availability of coal make it an attractive fuel for large, stationary operations such as power plants. However, coal suffers from many draw backs as a fuel. Compared to petroleum or natural gas, coal has a relatively low energy content, as well as a high moisture content, which reduces net energy conversion. The solid nature of coal may also make it difficult to transport perhaps when it is not near rail lines (e.g., not pipeline compatible), and it is not compatible with conventional engines. Further, coal may contain a significant mass fraction which is not fuel and can result in air pollution in the form of SOx, NOx, mercury, particulate, and other unwanted compounds. One way to add value to coal feedstocks, perhaps as well as address some of the pollution problems related to coal, may be to convert the solid coal into a liquid or gaseous fuel. This may be achieved through coal-to-liquid (CTL) technologies or perhaps even conversion to methane.
Coal is a low cost and abundant energy source which is vital to the United States. However, coal suffers from certain draw backs, such as the inability to use coal directly as a transportation fuel and perhaps the significant amount of pollutants that are released from the combustion of coal. Coal-to-liquid fuel and coal-to-methane technologies may allow coal to be converted to value-added products. However, these technologies currently are inefficient and economically uncertain.
Conventional CTL technologies may be based upon direct liquefaction or the Fischer-Tropsch reaction, in which coal may be gasified and subsequently reacted to produce olefins and paraffin. These technologies have existed for more than 80 years and have been shown to be operable at an industrial scale. CTL technologies have been used successfully in places such as South Africa, where coal is abundant but petroleum supplies can be tenuous. However, these technologies have never enjoyed wide popularity perhaps due to their inefficiency, environmental concerns, and uncertain economics. Some coal-to-liquid technologies may only be capable of producing 1-2 barrels of synthetic petroleum per ton of coal, and may require catalysts in addition to high temperatures and even high pressures for the reaction. The inefficiencies of these processes may result in a very large environmental footprint, perhaps with a typical release of over about 60% of the carbon input as CO2.
A factor that may have kept CTL technologies from gaining wide popularity may be the economic uncertainty from volatile oil prices. Current outlook projects that CTL may be economically viable perhaps when oil prices are greater than about $60 per barrel. Therefore, these technologies may be attractive with current oil prices (>about $100/bbl). However, CTL plants may require larger capital costs, and oil prices have dropped below the about $60/bbl mark as recently as 2009.
Another unconventional method of utilizing coal has been the production of biogenic methane. Microorganisms produce methane from coal under natural conditions in many coal formations around the world. In recent years, there has been significant interest in technologies for enhanced production or collection of biogenic methane.
Enhanced biogenic methane production may be achieved in the laboratory, and pilot scale experiments are currently under way such as in the Powder River Basin of Wyoming, China, Australia,—and elsewhere—to determine if these technologies are scalable and economically viable. While coal-to-biogenic methane processes may produce a value-added product from coal, methane prices are currently well below those of transportation fuels. If the United States continues to experience the current low methane prices, these technologies that produce enhanced biogenic methane may not be an attractive use for coal.